As a bearing device used for a spindle motor or the like of a hard disk device, a hydrodynamic bearing device which is more excellent in rotational accuracy than a ball bearing and also excellent in silentness is frequently adopted in place of a ball bearing device conventionally used.
As a hydrodynamic bearing device of this kind, there is a hydrodynamic bearing device disclosed in, for example, Japanese Patent Laid-Open No. 11-82486. The hydrodynamic bearing device includes a shaft 51, a sleeve 52 which is disposed at an outer periphery via a gap with respect to the shaft 51, and thick thrust flanges 53 and 54 disposed at both ends of the shaft 51 and in such postures as have gaps with respect to both end surfaces of the sleeve 52 as shown in FIG. 17, and a working fluid composed of lubricating oil is held in the gap between an outer peripheral surface of the shaft 51 and an inner peripheral surface of the sleeve 52, and the gaps between inner surfaces of the thrust flanges 53 and 54 (a lower surface of the thrust flange 53 and an upper surface of the thrust flange 54) and both the end surfaces of the sleeve 52 opposed to them. Dynamic pressure grooves 56 are formed on the outer peripheral surface of the shaft 51, and a radial hydrodynamic bearing in which the shaft 51 and the sleeve 52 are rotatably supported via a predetermined gap in a radial direction is constructed by the pressure of the working fluid collected by the dynamic pressure grooves 56 when the shaft 51 and the sleeve 52 are relatively rotated by a rotational driving force of a motor not shown. Dynamic pressure grooves 57 and 58 are formed on the inner surfaces of the thrust flanges 53 and 54, and a thrust hydrodynamic bearing in which the shaft 51 and the sleeve 52 are rotatably supported via predetermined gaps in a thrust direction (axial direction) is constructed by the pressure of the working fluid collected by the dynamic pressure grooves 57 and 58 when the thrust flanges 53 and 54 mounted to the shaft 51 and the sleeve 52 are relatively rotated by the above described rotational driving force or the like.
In the hydrodynamic bearing device, a plurality of communicating paths 59 extending in parallel with the axis are formed at each proper angle (for example, 180 degrees) around the axis at intermediate spots between the inner peripheral surface and the outer peripheral surface in the sleeve 52. The communicating paths 59 communicate with spaces between the inner surfaces of the thrust flanges 53 and 54 and both the end surfaces of the sleeve 52 opposed to them. Fluid closing members 60 and 61 are fitted in both end inner peripheral parts of the sleeve 52 so as to oppose to the outer peripheral surfaces of the thrust flanges 53 and 54 with gaps. Conical inclined surfaces 60a and 61a are formed at the areas of the fluid closing members 60 and 61 opposed to the communicating paths 59, and areas facing the inclined surfaces 60a and 61a are set as fluid storage spaces 64 and 65 in which the working fluid is stored. Between the outer peripheral surfaces of the thrust flanges 53 and 54 and the inner peripheral surfaces of the fluid closing members 60 and 61, the aforementioned gaps are formed and communicate with external air (atmospheric pressure), and fluid sealing parts 62 and 63 which seal the working fluid inside the hydrodynamic bearing device by utilizing the surface tension of the working fluid are provided.
Even when the pressure of the working fluid becomes unbalanced and a pressure difference occurs in the space between the outer peripheral surface of the shaft 51 and the inner peripheral surface of the sleeve 52 where the radial hydrodynamic bearing is formed, and in the spaces between the inner surfaces of the thrust flanges 53 and 54 and both the end surfaces of the sleeve 52 opposed to them, the pressure difference is eliminated by forming the communicating paths 59 as described above. Namely, even when the pressure of the working fluid becomes unbalanced by the construction provided with the communicating paths 59, the bearing function is stabilized and the working fluid is prevented from scattering outside by adjusting the pressure so as to eliminate the pressure difference in the working fluid.
In a general hydrodynamic bearing device of this kind, the gap where the radial hydrodynamic bearing is formed, and the gaps where the thrust hydrodynamic bearing is formed are extremely small, and therefore, in the operation of assembling the hydrodynamic bearing device and filling the working fluid into the hydrodynamic bearings, the working fluid is filled into the inside of the hydrodynamic bearing device so as to be favorably filled into the inside. However, with such an effort, a part of air sometimes remains in the space between the outer peripheral surface of the shaft 51 and the inner peripheral surface of the sleeve 52 where the radial hydrodynamic bearing is formed and in the spaces between the inner surfaces of the thrust flanges 53 and 54 and both the end surfaces of the sleeve 52 opposed to them where the thrust hydrodynamic bearing is formed. In addition, small bubbles are sometimes wrapped up and included in the working fluid when the hydrodynamic bearing device is rotating. When air is included inside as bubbles and attached to the dynamic pressure groove 56 of the radial hydrodynamic bearing and the dynamic pressure grooves 57 and 58 of the thrust hydrodynamic bearing like this, a feeding amount of the working fluid by the dynamic pressure grooves 56, 57 and 58 becomes small, and problem of reduction in bearing stiffness due to bubbles, and reduction in bearing performance such as instability of rotation at the time of rotational operation and the like are caused.